C23 makes various standard library functions, that return a pointer
into an input array, into macros that return a pointer to const when
the relevant argument passed to the macro is a pointer to const.  (The
requirement is for macros, with the existing function types applying
when macro expansion is suppressed.  When a null pointer constant is
passed, such as integer 0, that's the same as a pointer to non-const.)

Implement this feature.  This only applies to C, not C++, since such
macros are not an appropriate way of doing this for C++ and all the
affected functions other than bsearch have overloads to implement an
equivalent feature for C++ anyway.  Nothing is done to apply such a
change to any non-C23 functions with the same property of returning a
pointer into an input array.

The feature is also disabled when _LIBC is defined, since there are
various places in glibc that either redefine these identifiers as
macros, or define the functions themselves, and would need changing to
work in the presence of these macro definitions.  A natural question
is whether we should in fact change those places and not disable the
macro definitions for _LIBC.  If so, we'd need a solution for the
places in glibc that define the macro *before* including the relevant
header (in order in effect to disable the header declaration of the
function by renaming that declaration).

One testcase has #undef added to avoid conflicting with this feature
and another has const added; -Wno-discarded-qualifiers is added for
building zic (but could be removed once there's a new upstream tzcode
release that's const-safe with this C23 change and glibc has updated
to code from that new release).  Probably other places in glibc proper
would need const added if we remove the _LIBC conditionals.

Another question would be whether some GCC extension should be added
to support this feature better with macros that only expand each
argument once (as well as reducing duplication of diagnostics for bad
usages such as non-pointer and pointer-to-volatile-qualfied
arguments).

Tested for x86_64.

Add the C23 memset_explicit function to glibc.  Everything here is
closely based on the approach taken for explicit_bzero.  This includes
the bits that relate to internal uses of explicit_bzero within glibc
(although we don't currently have any such internal uses of
memset_explicit), and also includes the nonnull attribute (when we
move to nonnull_if_nonzero for various functions following C2y, this
function should be included in that change).

The function is declared both for __USE_MISC and for __GLIBC_USE (ISOC23)
(so by default not just for compilers defaulting to C23 mode).

Tested for x86_64 and x86.

Here is a patch updating the documentation to mention GNU and BSD
extensions that were adopted by POSIX.1-2024.

* manual/llio.texi (Memory-mapped I/O): Add that MAP_ANON and
MAP_ANONYMOUS were added by POSIX.1-2024.
* manual/memory.texi (Changing Block Size): Mention that reallocarray
was added by POSIX.1-2024.
* manual/message.texi (Message Translation): Adjust wording to match
standardization.
(Translation with gettext): Mention the gettext family of functions were
added by POSIX.1-2024.
* manual/pattern.texi (Wildcard Matching): Mention that FNM_CASEFOLD was
added by POSIX.1-2024.
* manual/process.texi (Creating a Process): Mention that _Fork and
WCOREDUMP were added by POSIX.1-2024.
* manual/signal.texi (Miscellaneous Signals): Mention that SIGWINCH was
added by POSIX-1.2024.
* manual/startup.texi (Environment Access): Mention that secure_getenv
was added by POSIX.1-2024.
* manual/string.texi (Truncating Strings): Mention that strlcpy,
strlcat, wcslcpy, and wslcat were added by POSIX-1.2024.
(Search Functions): Document that memmem was added by POSIX-1.2024.
* manual/terminal.texi (Allocation): Mention that ptsname_r was added by
POSIX-1.2024.
* manual/threads.texi (Waiting with Explicit Clocks): Move node under
POSIX Threads.  Mention pthread_cond_clockwait,
pthread_rwlock_clockrdlock, and pthread_rwlock_clockwrlock were added by
POSIX-1.2024.
(Joining Threads): New node under Non-POSIX Extensions.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Signed-off-by: Collin Funk <collin.funk1@gmail.com>

* manual/string.texi: For strnlen (s, maxlen), do not say that s must
be of size maxlen, as it can be smaller if it is null-terminated.
This should help avoid confusion such as seen in
<https://lists.gnu.org/r/bug-gnulib/2024-06/msg00280.html>.
Mention that strnlen and wcsnlen have been in POSIX since
POSIX.1-2008.

* manual/string.texi (String/Array Conventions):
Fix typo reported by Alejandro Colomar <alx@kernel.org> in:
https://sourceware.org/pipermail/libc-alpha/2023-November/152646.html

Co-authored-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

* manual/string.texi (Truncating Strings): Update obsolescent
reference and use the more-generic term “AddressSanitizer”.
Mention fortification, too.  -fcheck-pointer-bounds is no longer
supported.

* manual/string.texi: Editorial fixes.  Do not say “text” when
“string” or “string contents” is meant, as a C string can contain
bytes that are not valid text in the current encoding.
When warning about strcat efficiency, warn similarly about strncat
and wcscat.  “coping” → “copying”.
Mention at the start of the two problematic sections that problems
are discussed at section end.

Almost all uses of rawmemchr find the end of a string.  Since most targets use
a generic implementation, replacing it with strchr is better since that is
optimized by compilers into strlen (s) + s.  Also fix the generic rawmemchr
implementation to use a cast to unsigned char in the if statement.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>